Restriction indicators provide a signal indicating when it is necessary to service the air cleaner of an internal combustion engine. They typically are connected to the air outlet of the air cleaner upstream of the intake of the engine. Most mechanical restriction indicators employ a bellows-type diaphragm vented to the atmosphere, which is resisted from expanding by a compression spring. As the air cleaner becomes more contaminated, the differential pressure between the "atmospheric"/high pressure side and the "negative"/low pressure side of the restriction indicator increases, thereby causing the diaphragm to expand. When the differential pressure has reached a certain level, the diaphragm typically becomes locked in an expanded position, and a constant warning signal is provided to the user that the air cleaner should be serviced. An example of such prior art restriction indicators is shown in U.S. Pat. No. 3,443,365.
A significant concern with restriction indicators is the accuracy with which they indicate that a particular differential pressure has been reached. Restriction indicators are designed to provide a warning indication at a specific design pressure differential. It is important that they are accurate because improper indication would result in the air cleaner being serviced earlier or later than intended. It is difficult, however, to consistently manufacture accurate restriction indicators due to the variability of the component parts which make up the assembly. Component parts such as the spring, diaphragm, housing and locking mechanism, even when manufactured to close tolerances, can affect the accuracy of the indicator. Current designs, however, do not provide a mechanism for calibrating and adjusting the restriction indicator to account for these manufacturing variations.
Another problem associated with restriction indicators is providing a locking mechanism which will consistently lock and release after repeated use. In many designs, the locking mechanism includes a resettable plunger which engages a latch to lock the diaphragm in an expanded warning position. An example of this is shown by the '365 patent, in which the plunger is bent to engage the latch off-center.
What has been needed is a restriction indicator for air cleaner systems with a simple mechanism for permitting calibration to adjust for manufacturing inconsistencies. What has also been needed is a simple mechanism for locking a plunger in a restriction indicator.